Phase readout of thermal conductivity-based gas sensors

This work presents an integrated phase-domain ADC for surface-micromachined thermal-conductivity-based gas sensors. The concentration of hydrogen in air is detected via the concentration-dependent phase shift of an AC-driven thermal filter. Because thermal conductivity is measured in the phase, rather than in the amplitude domain, the readout circuit is insensitive to gain and offset errors. The ADC is implemented using a phase-domain sigma-delta modulator. Simulation results show that the system has a resolution of 0.06%_rms H₂ concentration in a 1Hz BW, while dissipating only 100μ;W in the thermal conductivity sensor.